1. Field of the Invention
The present invention relates to an ion beam apparatus for use in photolithography which directly exposes a photoresist formed on a semiconductor wafer, as well as in ion implantation, ion etching and so forth.
2. Description of the Prior Art
FIG. 3 schematically shows the structure of a typical ion beam apparatus. This apparatus is broadly divided into an ion gun a, a focusing lens system b and a deflecting electrode c. For example, the ion beam apparatus is employed to form a desired pattern on a sample with a generated ion beam and where the sample d which is the pattern is generally disposed on a support and a desired figure is drawn thereon by the use of an ion beam. The throughput in such operation is considerably affected by a probe current of the ion beam, and when it is required to increase the throughput by increasing the probe current, realization of a high luminance with a large ion current density is one of important requirements for the ion gun. In FIG. 3 are also shown an emitter 1', an extractor 2' and an aperture e.
FIG. 4 illustrates a conventional ion gun, which comprises a needle emitter 1 with a pointed tip having a radius of curvature of 500 to 1000 .ANG., and an extractor 2 which has a hole of 1 mm in diameter. The needle emitter 1 is attached to a fore end 10 of a refrigerator with an insulator 9. When a high voltage is applied to the needle emitter 1, a high-intensity electrical field is generated selectively at its tip to ionize atoms (molecules) of a gas such as helium which is delivered via a gas supply pipe 4 to fill the periphery of the emitter 1. (Normally the applied voltage ranges from 20 to 30 kV.)
The ions thus obtained form an ion beam which decrease radially from the tip of the emitter. In the ion beam generated at the emitter tip, the density of the atoms (molecules) existing in the vicinity of the emitter can be maintained high by cooling the emitter, so that it is possible to obtain an ion beam having a high current density.
The probe current of the focused ion beam is required to have a value of 10 to 100 pA in practical use. For satisfying such requirement, the gas pressure in the ion gun must be in the order of 10.sup.-3 Torr, because the relationship of FIG. 5 exists between the helium pressure and the ion current, which shows that the helium pressure must be raised to increase the ion current. Therefore during patterning with an ion beam, for example, a high helium gas pressure is required to maintain a sufficient amount of current.
For keeping the helium gas pressure in the order of 10.sup.-3 Torr or so under conditions where the vacuum degree outside the ion gun (i.e. in a bell jar) is 10.sup.-5 Torr, a great amount of helium gas must be supplied. However, increasing the amount of the supply helium gas causes a temperature rise in the emitter, which lowers the density of atoms (molecules) in the vicinity of the emitter and to consequently reduce the ion beam current density. Although the emitter is cooled as mentioned above to attain a high ion beam current density, there is a temperature limit of about 4.degree. to 10.degree. K. for cooling. Since it is customary to supply helium gas at room temperature into the ion beam apparatus, when the amount of the supply helium gas is increased as shown in FIG. 6, there is a temperature rise in the emitter 1. As a result, if the amount of the supplied helium gas is increased for the purpose of raising the gas pressure, the ion beam current density tends to become lower due to such temperature rise in the emitter to eventually cause a problem in that an expected increase of the probe current is not achieved.
Although the emitter temperature may be maintained at a sufficiently low point by enhancing the cooling capability for the increased amount of the supply helium gas, a disadvantage is unavoidable in that the procedure for such enhancement of the cooling capability requires that system be far more expensive than the present system.
Besides the above, the gas pressure between the emitter and the extractor is also raised with increased helium gas pressure, so that the problem of atmospheric electrical discharge becomes serious. Such atmospheric electrical discharge causes breakage of the emitter tip, which therefore fails to perform the proper function of an ion gun.